The overall objective is to understand the mechanism of certain aspects of cell-mediated immunity, especially those pertaining to the interaction of sensitized lymphocytes and antigen leading to macrophage activation. This process appears to be important in immunity to microorganisms and tumors and may also underlie certain pathogenic states. In this application, we propose to continue and extend ongoing work on lymphocyte mediators and their relation to macrophage activation using a mouse model. Specifically: 1) further studies on the purification and characterization of murine MIF; 2) compare the properties of the in vitro induced mediator with murine MIF induced in vivo by intravenous antigen challenge of an appropriately sensitized animal; 3) further characterization of bone marrow-derived macrophages, including studies on the mechanism of their activation by lymphocyte mediators; 4) characterize further factors which modulate the graft vs host reaction in the mouse.